This invention has particular utility in a radar receiver carried on board an air-to-air missile. Analog center-line filters are conventionally used in the receiver, which has several channels, each requiring a center-line filter. The center-line filters must be nearly identical in performance. Digital sampling at frequency f.sub.s of continuous signals results in the digital data values being the same for harmonics of f.sub.s. For example, one cannot tell the difference between 0.9 f.sub.s, 1.9 f.sub.s, 2.9 f.sub.s, 3.9 f.sub.s, and so on. This is called aliasing, or folding, or ambiguity of the spectrum. The problem occurs in two typical signal or multi-tone cases, when a high frequency interfering signal folds (or aliases) back into the spectrum near or on the desired signal. An anti-aliasing filter eliminates the potential aliasing frequencies before digital sampling. Then the user knows the true frequency of the signal. A classical center line filter is like a super anti-aliasing filter because it only passes a very narrow region of frequencies of interest. Therefore, after digital sampling, there is no spectral aliasing.
Space (size) considerations constrain the design of analog filters, and the slow roll-off of analog filters make this type of filter unacceptable as an anti-aliasing filter. Moreover, analog filters are relatively expensive. Conventional analog center-line filters operate at base band, and moving the frequency band to higher IF would be difficult as the Q of the filter may be too high. Further, it is difficult to achieve channel matching requirements due to frequency drift.